This invention relates generally to machines having an element movable along guideways, such as a coordinate measuring machine, and more particularly to coordinate measuring machines that can accommodate materials having different coefficients of thermal expansion while maintaining a high level of accuracy and stability at different temperatures.
Coordinate measuring machines are used for dimensional inspection of workpieces, such as machined or molded parts. A workpiece typically is secured to a fixed table and a measuring probe is secured to a ram which is movable in three dimensions. To measure the position of a point on the workpiece, the probe is brought into contact with the point, and measuring scales or other sensors on the machine are read. The position of the point is typically expressed in X, Y and Z coordinates within a working volume of the machine. To measure a distance between two points, the points are contacted successively, the coordinates of both points are read, and the distance is calculated from the coordinates. State of the art coordinate measuring machines typically have features such as high resolution measuring systems, electrical contact probes, motor drives, computer controlled drives and computer acquisition and processing of data.
Two common types of coordinate measuring machines are a moving bridge machine, and a gantry style machine. In both, a bridge moves in the Y direction along guideways on a table or support. A carriage moves in the X direction along guideways on the bridge. A ram with a probe mounted on its lower end moves vertically or in the Z direction through bearings in the carriage. Thus, three-dimensional movement of the probe is provided. The scales associated with each of the movable elements indicate the positions of the movable elements in the three axial directions.
The accuracy of a coordinate measuring machine is limited by geometric errors, such as inaccuracies in the scales or other measuring devices, and faults in the guideways or other elements which define machine motion. These inaccuracies can cause measurement errors when the machine is operated at a reference temperature, which is usually 20xc2x0 C. One known approach to increasing accuracy with respect to the geometric errors is to improve the construction techniques and to reduce tolerances of the system so that errors are reduced. However, reduction of errors becomes progressively more expensive as required accuracies increase. Another known approach is direct measurement of coordinate errors at points throughout the machine working volume. This approach is impractical because of a huge amount of data which must be stored for large machines and because of the time required to measure such data. In one example, U.S. Pat. No. 4,884,348 discloses a testing device for determining measurement errors.
A third known approach is the measurement of errors in parametric form. As noted, a coordinate machine typically has three sets of guideways which establish probe motion. Ideally, movement along each of these guideways should result only in linear motion, and a scale reading should equal the linear displacement. In reality, however, there are scale errors and the guideways are not completely straight or perfectly free from twists. For such a machine, there are six degrees of freedom which produce errors during movement along each guideway. For each direction of movement, there are three linear errors, DX, DY and DZ and three rotational errors AX, AY and AZ. The six error parameter is measured at a number of points along each direction of machine movement, resulting in an error matrix with 18 error parameters. From the matrix of 18 error parameters, the error at any point in the measurement volume is calculated and stored. The calculated errors are then subtracted from the measured coordinate values to determine actual workpiece coordinates. Examples of this approach are found in U.S. Pat. Nos. 4,884,889 and 4,939,678.
In addition to the above mentioned so-called geometric errors, the accuracy of a coordinate measuring machine is in general also affected by thermally induced errors that may cause deformation of machine components. These are errors resulting from thermal expansion, or differential thermal expansion, due to differing coefficients of thermal expansion of different machine components. It is well known that these measurement errors may be minimized by maintaining a coordinate measuring machine at a constant temperature to prevent changes in size of the various components due to thermal expansion or contraction. However, it is not always possible to maintain the environment surrounding a coordinate measuring machine at a constant temperature. This is particularly true in a shop environment where temperatures and humidity conditions will change from season to season and day to day.
In a manner that is analogous to the correction of geometric errors, the thermal errors can be minimized either by applying appropriate design techniques or by using software error compensation techniques. The latter method is based on taking real-time readings from temperature sensors mounted on the measuring machine and the use of a model that resides inside the machine""s controller. The model relates the sensor readings to geometric deformations caused by temperature changes. Correction values for the scale readings are calculated to offset these errors.
In the former method, various construction techniques may be used, such as employing materials with low coefficients of thermal expansion, or materials which all have the same coefficient of thermal expansion. Examples of such construction techniques are found in the following patents and applications: U.S. Pat. Nos. 4,538,911; 5,173,613; 5,198,874; 4,962,591; and 5,031,331; and Publication Nos. WO 89/09920 and WO 89/09887.
It is not always practical to make all components of the same material, or materials having the same coefficients of thermal expansion, since each component serves a different function and therefore should have properties that are different from other components. For example, it is desirable that the bridge be strong, but not necessarily heavy so that the bridge has a relatively low inertia. On the other hand, the guideways upon which the bridge and ram travel must be strong and formed of a material such as steel which provides a precision pathway. If the bridge were made of steel like the guideways, it would be too heavy to be of practical use and likely would be too expensive.
Most known prior art systems for fully temperature compensating a coordinate measuring machine are relatively complex and expensive. Therefore, it is desirable to have a coordinate measuring machine which can be used in various temperature environments and formed of materials of different coefficients of thermal expansion, and yet still possess a very high level of accuracy without the need for an expensive thermal compensation system.
This invention relates generally to machines having two elements movable with respect to one another along guideways such as rails, and to the compensation of thermally-induced errors that may cause deformation of components of the machines as a result of temperature changes, because of differences in the coefficient of thermal expansion. This invention has particular applicability to coordinate measuring machines.
In one aspect, this invention discloses a machine which includes a support structure, an element movable with respect to the support structure, at least one guideway disposed on one of the support structure and the movable element along which the movable element and support structure move relative to one another in a direction of travel, the guideway having a coefficient of thermal expansion which is different from that of the surface on which it is mounted, and at least one member disposed on the same structure on which the guideway is mounted generally opposite the guideway, the member being generally parallel to the guideway and having a coefficient of thermal expansion, a stiffness, a spacing from a center of mass of the structure on which the guideway is mounted, and a cross-sectional dimension such that the member substantially balances any thermal stresses created by differential expansion and contraction of the guideway and the structure on which the guideway is mounted for a given change in temperature to minimize any deformation of the structure on which the guideway is mounted. In one embodiment of this aspect, the member has the same coefficient of thermal expansion as the guideway. In another embodiment, the member has substantially the same cross-sectional dimension and length as the guideway. In yet another embodiment, there is a second member and a second guideway, and the one member and the one guideway are disposed generally opposite one another, while the second member and the second guideway are disposed generally opposite one another. In yet another embodiment, the guideway is disposed on one surface of the support structure and the member is disposed on a second surface of the support structure that is aligned generally parallel to the one surface. In yet another further embodiment, the guideway is disposed on one surface of the support structure, and the member is disposed on a second surface of the support structure that is aligned generally parallel to the one surface.
In yet another further embodiment of this aspect, the member is formed of the same material as the guideway, and the member has substantially the same cross-sectional dimension, the same stiffness, and the same spacing from the center of mass of the structure on which the member is mounted as the guideway. The member may be affixed by an epoxy, or by screws, or both. In yet another embodiment, the member is disposed on a side of the center of mass of the structure on which the member is mounted that is opposite the side of that structure on which the guideway is disposed. The machine may be a coordinate measuring machine, and the support structure may be a beam movable in one direction, and the movable element may be a carriage carrying a Z ram movable in a second direction orthogonal to the first direction. In this embodiment, the member and the guideway are mounted on the beam. In another embodiment, the machine is a coordinate measuring machine in which the support structure is a carriage, the movable element is a Z ram movable in a vertical direction, and the guideway and member are mounted on the Z ram.
In another aspect, a coordinate measuring machine includes an elongated beam movable in one direction generally perpendicular to its direction of elongation, at least one rail that is disposed on the beam and extends in a direction generally parallel to the direction of elongation of the beam, the rail being formed of a material having a coefficient of thermal expansion different from that of the beam, a carriage that is movable along the rail in the direction of elongation of the beam, and a bar that is disposed on the beam generally on an opposite side of a center of mass of the beam from the rail, the bar having a coefficient of thermal expansion, a cross-sectional dimension, a stiffness and a spacing from the center of mass of the beam such that the bar substantially balances any thermal stresses on the beam produced by differential expansion or contraction of the beam and the rail with changes in temperature.
In yet another aspect of the invention, a coordinate measuring machine is disclosed which includes a carriage movable along a beam in a first direction, a ram movable with respect to the carriage in a second direction generally orthogonal to the first direction, the ram having a rail extending in the second direction disposed on one side of the ram and along which the ram travels with respect to the carriage, the rail having a coefficient of thermal expansion different from the coefficient of thermal expansion of the ram, and a bar extending generally parallel to the rail and disposed on a side of the ram generally opposite of the one side, and generally opposite of the center of mass of the ram from the rail, the bar having a coefficient of thermal expansion, a cross-sectional dimension, a stiffness and a spacing from the center of mass of the ram such that the bar substantially balances any thermal stresses on the ram produced by differential expansion or contraction of the ram in the rail with changes in temperature.
In yet another aspect of this invention, a coordinate measuring machine is disclosed which includes an element movable in a first direction, a rail along which the element travels, the rail extending in the first direction, a support structure on which the rail is mounted, the support structure having a center of mass, a member disposed in the support structure on a side of the center of mass of the support structure opposite of the rail, the member having a coefficient of thermal expansion, a cross-sectional dimension, a stiffness and a spacing from the center of mass of the support structure such that the member substantially balances any thermal stresses on the support structure produced by differential expansion or contraction of the support structure with changes in temperature.
In yet another further aspect, a machine is disclosed which includes a beam movable in a first direction along a rail assembly, a slide coupled to the rail assembly which permits the beam to travel along the rail assembly in the first direction, a slot disposed on the slide that is elongated in a direction generally perpendicular to the first direction, and a pin affixed to the beam and extending into the slot, whereby any expansion or contraction of the beam in a direction perpendicular to the first direction causes the pin to move in the slot in the direction perpendicular to the first direction.
In yet another further aspect, a machine is disclosed which includes a beam, two generally parallel rails disposed on the beam, the rails extending in the first direction, a carriage movable along the rails in the first direction, slides associated with each of the rails for permitting the carriage to move along the rails, and a flexible coupling between the carriage in at least one of the slides to permit the carriage to move with respect to the slide in a second direction generally perpendicular to the first direction. In one embodiment, the carriage is supported by the coupling in a spaced relationship with the slide. In another embodiment, the coupling allows movement of the carriage in the second direction with respect to the one slide, but is sufficiently rigid in a direction perpendicular to both the second direction and the first direction to maintain the carriage in a spaced relationship with the slide. In yet another embodiment, the coupling is a leaf spring. This machine may be a coordinate measuring machine.
In yet another further aspect, a method is disclosed of balancing thermal stresses on a beam having a rail along which an element moves with respect to the beam, the rail and the beam having different coefficients of thermal expansion, the method comprising determining a center of mass of the beam, mounting a member on the beam on a side of the center of mass of the beam opposite from a side of the center of mass of the beam on which the rail is disposed, the member being mounted such that it extends generally parallel to the rail, and selecting a size and material and stiffness for the member such that a coefficient of thermal expansion of the member, a stiffness of the member, a cross-sectional area of the member and a spacing of the member from the center of mass of the beam causes the member to substantially balance thermal stresses in the beam produced by differential expansion or contraction of the beam with respect to the rail with changes in temperature. In one embodiment, the member is mounted on the beam at a distance from the center of mass of the beam which equals a distance from the center of mass to the rail, the coefficient of thermal expansion of the member, the stiffness of the member and the cross-sectional area of the member are all equal to the respective coefficient of thermal expansion, stiffness and cross-sectional areas of the rail.
In yet another aspect of the invention, a method of minimizing any bending of a beam in a coordinate measuring machine is disclosed, the beam having a rail thereon, the beam and the rail having different coefficients of thermal expansion, the method including the step of determining a center of mass of the beam, placing a bar on the beam on a side of the center of mass of the beam opposite a side of the center of mass of the beam on which the rail is disposed, aligning the bar in a direction generally parallel to the rail and selecting a cross-sectional area of the bar, a material for the bar having a coefficient of thermal expansion, a spacing of the bar from the center of mass of the beam and a stiffness of the bar such that any stresses produced in the beam by the rail as a result of changes in temperature are generally equal to the thermal stresses produced in the beam by the bar. In one embodiment, the bar is affixed to the beam so that the bar cannot move with respect to the beam. In another embodiment of this aspect, a coefficient of thermal expansion, stiffness, spacing from the center of mass and a cross-sectional area are all selected for the bar so that they are the same as the respective coefficient of thermal expansion, stiffness, spacing from the center of mass of the beam and cross-sectional area of the rail.